Problems associated with various types of equipment are often characterized by the generation of noise, including components thereof which are in the ultrasonic range of frequencies. Such problems include corona discharges, worn bearings and valves, high pressure fluid leaks, and the like. While it is often possible to find and diagnose such problems from the noise made thereby which falls in the human audible frequency range, such location and diagnosis of the problem is difficult when the equipment is in an already noisy environment. Thus, apparatus have been developed and employed which are sensitive to ultrasonic frequency sound for use in such noisy environments.
The ultrasonic detectors employed include generally a microphone sensitive to ultrasonic frequencies, a high pass filter to reject audible frequency sound, and means to indicate the detection of ultrasonic noise. Such means may include a heterodyne arrangement, wherein a local oscillator signal is combined with the received ultrasonic signal to convert same to an audible frequency or an envelope detector or A.M. (amplitude modulation) type of detector to detect the variations in the amplitude of the ultrasonic signal. The detectors may further include a signal strength meter, audio amplifiers and filters, a power amplifier, and a speaker.
In the telephone industry, communication cables are often pressurized with an inert gas to preclude the entrance of moisture which could erode the electrical isolation between the various conductors of the cable. When a gas leak occurs in such a cable, the use of ultrasonic detectors to find the leak is often laborious and time consuming because of the highly directional nature of sound of ultrasonic frequencies and because of the low intensity of ultrasonic sound from the relatively low pressure gas leak.